Highly radiogenic crustal Pb in the cobalt-copper ores of central Idaho,
USA : Implications for ore genesis
Lead (Pb) isotopes have been widely used as tracers of metals in ore genetic studies.
Because of similar chemical behavior during transport and deposition it works well for
Zn, Ag, and to some extent Cu deposits. In addition, the Pb-bearing mineral galena is
often an important ore mineral in many of the Zn, Ag, and Cu deposits. However, for
many other metallic mineral deposits using Pb as a proxy may not be appropriate and
may not serve the purpose of tracing the sources of metals.
Cobalt-copper ores of the Blackbird mine occur in the metasedimentay rocks of the
Yellowjacket Formation (Middle Proterozoic) in central Idaho. The mjor ore minerals are
cobaltite, chalcopyrite, pyrite, and arsenopyrite. Both magmatic hydrothermal and
volcanogenic origins for the ores have been proposed. The timing of mineralization is
only speculative at present and varies with the perceived ore genetic model. Many
investigators favor a Proterozoic syngenetic model related to mafic volcanism because of
the enrichment of ore suite elements (Fe, As, Bi, Au, Co, Cu) that are typically found in
mineral deposited related to mafic magmatism (Nash and Gregory, 1986).
However, the Pb isotope data (fig. 1) on ore minerals, host rock, and rocks outside the
cobalt belt indicate that at least the Pb was derived from the host metasedimentary
sequence, not from the underlying crystalline basement rocks or from any mafic igneous
sources. This idea is also validated by somewhat heavier sulfur isotope compositions of
the ore minerals (d 34S, 6.6-8.1 ‰). In the absence of mineralization ages, the geologic,
chemical, and isotopic characteristics could be explained in more than one way: 1) the
cobalt-copper ores formed by volcanic exhalites contemporaneous with the deposition of
host Yellowjacket Formation and Pb was introduced by hydrothermal fluids during a
subsequent thermal event such as the emplacement of the Idaho batholith, 2) the cobalt-
copper ores formed by hydrothermal leaching of a sedimentary sequence mixed with
volcanic rocks at a later time thus obtaining ore suite elements from mafic volcanics and
Pb from metasedimentary rocks. In either case, a volcanic source of ore metals and
mixing of Pb from sedimentary and volcanic sources are implied. Since the volcanic
rocks are only inferred to be present in the metasedimentary sequence, the third
possibility is that the ores formed by hydrothermal fluids leaching sedimentary rocks
originally containing metal rich nodules. The manganese nodules of the sea floor are
known to be rich in Co and leaching of such a source could lead to the formation of
epigenetic cobalt ores.
Cobalt-copper ores of central Idaho could be an example of a case where Pb
isotopes alone may not be able to unlock the secret of ore metal sources. The
inconsistency between the ore genetic models based on trace elements (volcanogenic)
and Pb isotopes (hydrothermal leaching) may be partly attributed to the metamorphosed
nature of the mineral deposit.
Origin of Cobalt-Copper Ores of Central Idaho, USA – A Pb isotope
A sedimentary source of Pb can be reconciled with other geochemical characteristics
indicative of volcanogenic sources by means of a mixed hydrothermal system (volcanic
and sedimentary) operating contemporaneously with the formation of the host
The Mesoproterozoic Yellowjacket Formation of central Idaho is considered as
the stratigraphic equivalent of the Belt Supergroup (Evans and others, 2000, Link and
others, 1980). The Belt Supergroup being the classic Mesoproterozoic stratigraphic
section of North America, continues to receive attention from a wide variety of
geologists. The Belt Supergroup and its stratigraphic equivalents in the United States
(Idaho, Montana, Washington) and Canada (British Columbia) host some of the world
class mineral deposits like Sullivan (Canada) and Coeur d’Alene (northern Idaho), as
well as several small to medium size deposits like the Blackbird and Iron Creek (cobalt-
copper-iron) in central Idaho. The Yellowjacket Formation marks the southern limit of
the Belt basin (Fig. 1). The Black Bird mine is the largest known cobalt deposit in the
United States (Roberts, 1953). Neither the age of the Yellowjacket Formation nor the
origin of ores hosted by them have been equivocally established at present. In this paper,
we present 14 new lead (Pb) isotope data on silicate rocks and sulfide ores and 2 sulfur
(S) isotope data on sulfide ores. The results are discussed in combination with
information available from published work to provide new insights into the age of the
Yellowjacket Formation and the origin of cobalt-copper ores.
Location: longitude and latitude
Blackbird mining district includes several mines which are part of the Idaho cobalt belt.
Similar to Ducktown Cu-Zn deposit – high in Fe, has characteristics of volcanogenic
exhalative ores, but occurs in overwhelmingly sedimentary environment. A mixed
sedimentary-igneous origin presented by LeHuray (1984) based on Pb and S isotopes.
Assuming that the Pb isotope compositions of the sulfide ores did not change
considerably since its formation, and knowing the isotope composition of the silicate
rocks at the time of their formation, some constraints can be placed on the potential
sources of Pb in the sulfide ores and on the timing of mineralization.
We envision leaching of Pb and other metals by a hydrothermal fluid (basinal brine?)
from the host sedimentary sequence and subsequent deposition of ores in structurally and
geochemically favorable sites as the major process. No leaching experiment was
conducted on rocks of the Blackbird district during this study. However, leaching studies
conducted on rocks from other parts of Idaho (Panneerselvam, 2001) and on rocks from
South America (Kamenov et al., 2002) generally produced leachates with higher Pb
isotope values than the whole rock. It is also known that highly saline fluids are more
efficient in leaching metals from silicate rocks. High salinity fluid inclusions? and high
chlorine content of wall rock biotite (?) reported by Nash and Connor (1993) are in
concert with the hydrothermal leaching model proposed in this report. The timing of the
mineralization can not be determined precisely based only on Pb isotope compositions
but some broad constraints can be placed. Assuming that the Pb isotope compositions of
the sulfide ores did not change considerably since its formation, and knowing the isotope
composition of the silicate rocks at the time of their formation, some constraints can be
placed on the potential sources of Pb in the sulfide ores and on the timing of
Bennett (1977) suggested that the deposit was originally stratabound and later
remobilized during metamorphism
Anderson (1943), attributed the mineralization to a deep magmatic source with solutions
permeating fractured host rock.
Age of the Yellowjacket Formation
Evans and Zartman (1990) reported 1370 Ma for a granite that intruded the
Yellowjacket Formation, providing a lower limit on its age. Traditionally, the
Yellowjacket Formation is correlated with the lower Belt Supergroup, and some
investigators even think that Yellowjacket Formation is older than the Belt Supergroup
based on stratigraphic correlations. However, our results indicate a much younger age, as
will be discussed later.
The term Yellowjacket Formation (unrestricted) is used in this report to include
all metasedimentary of the region, which includes Salmon River Mountains and the
Lemhi Range. Ross (1934) originally used the term Yellowjacket Formation to strata
beneath the Hoodoo Quartzite in the vicinity of Yellowjacket mine, a few miles west of
the Blackbird mine, in the Salmon River Mountains. Lower units of the Yellowjacket
Formation are correlated with the Apple Creek Formation in the Lemhi Range (Tysdal,
2000). Under the revised stratigraphic nomenclature, the host rocks of the Blackbird mine
are part of the Apple Creek Formation (Lemhi Group).
However, 1576 Ma old gneissic rocks are known to occur in the Priest River
complex in northern Idaho (Evans and Fischer, 1986).
Origin of cobalt-copper ores
Sullivan deposit – sediment-hosted, massive sulfide, Mesoproterozoic, Purcell basin,
Canada, classic sedex type
Pb-isotopes discussed by Beaudoin, 1997; 2000, ore Pb very unradiogenic, no rock Pb
Create a diagram showing Sullivan (sedex), Coeur d’Alene (metamorphic?),
Highly radiogenic nature of the Pb suggests upper crustal sources, not sources of mantle
affinity. Mafic volcanic rocks tend to have less radiogenic Pb indicative of mantle
sources. Given the very low Pb isotope ratio for the mantle (for instance, 206/204 <
17.00) and very high ratios for the upper crust (for instance, 206/ 206 > 19.00), mixing of
Pb from mafic volcanic sources (mantle derived Pb) and Proterozoic crustal sources
would produce a very broad range of Pb isotope data, such a range is not observed.
Neither a bimodal distribution showing one group of ores with mantle source and the
other group with upper crustal source is observed in the Pb isotope data. If the Pb and
other metals were derived from the same source, then the Co-Cu mineralization must
have occurred prior to the intrusion of any mafic igneous bodies. Previous geologic
studies indicate that the inferred mafic bodies are volumetrically minor compared to the
compared to the metasedimentary rocks (Nash..).
Where do Co-Cu (Au+ Fe) come from?
Transition metals: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
Siderophile elements: Fe, Co, Ni, Pt, Re, Os
Chalcophile elements: Cu, Ag, Zn, Pb, S
Ore suite elements (Fe, As, Bi, Au, Co, Cu) – Blackbird
1. Ore bodies are associated with biotite-rich wall rock.
2. Sediment-hosted strata-bound Fe-Co-Cu-Au deposits
3. Ore minerals magnetite, heamatite, cobaltite, pyrite, pyrrhotite, chalcopyrite
4. Ore textures and paragenetic sequence
6. Enrichment of ore suite elements (Fe, As, Bi, Au, Co, Cu)
7. High Cl in biotite?
8. High salinity fluid inclusions
9. Metamorphosed ore bodies
10. High REE and Y concentrations
11. Extremely radiogenic Pb in ores and rocks
12. Moderate sulfur isotope values
13. Behavior of Pb relative to Co and Cu during leaching, transport, and deposition
Slack (2006) assigns the deposits of the Blackbird district to the IOCG group based on
the association of ore metals and high concentrations of rare earth elements (REE) and Y.
The iron oxide-copper-gold (IOCG) is a grouping of ore deposits based on geochemistry
and does not imply any particular source or a specific process (Williams et al., 2005).
Nash and Connor (1993)
Regional metamorphism (greenschist facies) occurred prior to the emplacement of 1370-
Ma granitic pluton (Evans and Zartman, 1990)
Thinly bedded argillite, siltite, quartzite, and minor marble
Contains distinct interbeds of biotite-rich rock
Mafic strata outside of the mine area contain 5-30% scapolite porphyroblast, a mineral of
Mineraology and major element chemistry of metamorphosed mafic sills and dikes are
similar to that of the mafic strata (Nash and Hahn, 1989)
Fe-rich layers are chemical sediments, so are the silica rich layers (exhalatives)
Iron in rocks ranges from 10 to 35 percent and resides mainly in magnetite, hematite,
pyrite, and biotite
The Fe-silicate facies (biotite-rich zones) is enriched in Cl and suite of elements (Cr, Fe,
Mg, P, Sc, TI, V, Y, and Yb) charateristic of mafic rocks,
The magnetite-rich oxide facies is enriched in Cu, Sb, As, Ba, Pb, Se, and Zn, but only
rarely enriched in Co, a similar unit also occurs 50 Km southeast of Blackbird district.
Supersaline fluid inclusions containing up to six daughter minerals (Nash and Hahn,
1989) in coarsegrained quartz associated with sulfide minerals, homogenization
temperatures 275-375 C, salinity in wt % Nacl ?
Boron isotope analysis of tourmaline does not indicate a marine evaporite source for the
B. Moderate sulfur isotope values (6-8) of the sulfide minerals do not favor a marine
evaporite source of sulfur. Caution must be exercised in drawing inferences on the source
of sulfur, because a variety of factors like pH, fO2, T, total sulfur content of the fluid can
cause variations in the S-isotope values of sulfide minerals (Ohmoto, 1989?).
Mesoproterozoic seawater sulfate 17 +/- 3 per mil, Strauss, 1993 ?
High fluid-rock ratios, Upward moving overpressured diagenetic pore fluids vented to
seafloor, similar to Salton Sea geothermal system to explain the high salinity of the fluid
inclusions, and high chlorine content of biotite and other minerals.
An increase in the grade of metamorphism from greenschist facies (biotite zone) in the
southeast to amphibolite facies (sillimanite zone) towards the northeast along the Idaho
cobalt belt has been reported by Nold (1990). Accompanying the increase in
metamorphic grade is the change in ore mineral assemblage from pyrite-arsenopyrite-
chalcopyrite through cobaltite-pyrrhotite to chalcopyrite-pyrite-arsenopyrite, from
southeast to northwest.
Stratiform nature of the mineralization and the occurrence of ore bodies at the same
stratigraphic level over a strike length of 50 km are commonly invoked as evidences of
syngenetic mineralization (Nold, 1990).
Sheep Creek copper-cobalt deposit, Montana, in the main Belt basin.
Early exhalative hydrothermal, two sources of sulfur – seawater sulfate (massive py) and
magmatic (cpy), based on fluid inclusion and s isotope
Ref: Zieg and Leitch (1998)
Endowed with metals
Lindsey, Tysdal, and Taggart
Mesoproterozoic Lemhi Group comprises Big Creek, Apple Creek, and Gunsight
Formations in the Salmon River Mountains and the Lemhi Range in central Idaho.
Derived from Archean quartzofeldspathic gneiss (Dillon Granite Gneiss of southwestern
Montana). The lower and middle units of Connor (1990) are placed in the Apple Creek
Formation (Tysdal, 2000). The upper unit of the Yellowjacket Formation has been
assigned to Gunsight Formation by Tysdal (2003).
Tydsdal (2000) restricts the name Yellowjacket Formation to rocks originally described
by Ross (19 ), which includes part of the lower unit of the previous workers (Connor )
Chlorine concentrations as high as 1.87 weight percent were found in biotite that are
spatially associated with Co-Cu ores at Blackbird (Nash, 1989). Rocks are also rich in Cl.
Highly saline, cl-rich pore fluid may have played a role, Cl, K, B added probably during
diagenesis; suggest multiple hydrothermal stages and multiple sources of various
Could the biotite be the result of hydrothermal alteration of wall rocks (sediments) by Fe-
rich fluid that also carried Co and Cu?
Nash, Hahn, and Saunders Report 87-410
Gold associated with silicious rocks (metachert), late phase exhalations, up to 20 g/t,
Layers rich in Si, Fe, As, B, Bi, Co, and Nb
Biotite-rich layers are interpreted to be mafic tuffs
Influx of hot hydrothermal fluid (exhalative) from deep onto the sedimentary basin and
mixing with connate fluids evolved in the basin could explain many of the characteristics
of the Co-Cu ores. While a hot deep source of fluid is not required by the Pb isotope data,
it might help to explain the volcanogenic nature of the deposits.
Bending and Scales, 2001
Formation Capital Corporation U.S., 1995-96, Idaho cobalt project, includes four
deposits – Ram, Sunshine, East Sunshine, and Northfield. Feasibility studies of Ram and
Sunshine deposits estimate 2119280 tones of ore grading 0.683% Co, 0.54% Cu, and 0.02
oz per tone Au. Total resource base exceeding 5 million tones in four deposits. 18 Co-Cu
deposits in the belt; thickness of YJKT reaches 7600 m locally; dominant structures are
north- to northwest-trending faults and shear zones; growth faults conduits for fluids
circulating through the basinal sedimentary sequence; emplacement of gabbroic dikes
concurrent with the submarine hydrothermal activity; gold upto 20 g/t
Iron Creek – bedded magnetite below cabaltiferous pyrite, variable cp
Black Pine – bedded cp with a variable cobaltiferous arsenopyrite and minor cobaltite
Blackbird – bedded cobaltite with variable cp and native Au
Mineralization typically hosted by siliceous exhalites or biotite tuffaceous exhalite and
medium grained quartzites. Sedimentary disruption features
Very high Pb isotope values of the Blackbird ores suggest that Pb was derived from the
enclosing sedimentary sequence with similar values and not from any igneous source of
any age. While there is no Pb isotope data on the mafic rocks of the Blackbird mining
district, Proterozoic mafic rocks of the adjoining Belt-Purcell basin, the Moye sills,
contain much less radiogenic Pb isotope values (Beaudoin, 2000?) than the Blackbird
ores. The Cretaceous Idaho Batholith and Tertiary Challis volcanic and their equivalents
in central Idaho also contain Pb less radiogenic than the ores of the Blackbird district.
The widely held view, supported by this study, that the mineralization occurred during
Mesoproterozoic also rules out any Cretaceous or Tertiary rocks as sources of ore
components. Several studies have also concluded that mineralization preceded the
Proterozoic metamorphic event, hence a metamorphic hydrothermal system does not
appear to a major player in the formation of these deposits, while the metamorphic event
might have modified the configuration of the ore bodies and changed the mineral
assemblages to some degree.
A sedimentary source of Pb does not imply that all ore components originated from the
same source. Highly evolved hydrothermal systems can homogenize components derived
from various sources through which they migrate. Mixed source of ore components
allows the presence sedimentary Pb, and potentially other metals, and explains the
geochemical characteristics indicative of volcanogenic sources. Our limited sulfur isotope
data is consistent with such a mixed source model. Whether there were originally two
distinct hydrothermal fluids, one from volcanogenic or deep-seated source and the other
basinal brine, cannot be resolved by Pb isotopes alone. However, supersaline fluid
inclusions in quartz associated with sulfide ores and high chlorine content of biotite from
the wall rocks indicates the involvement of highly saline fluids in the formation these
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